Add and pre-enable a systemd service for powertop --auto-tune

On my system this saves about 0.4W while on battery power.

The major problem with `powertop --auto-tune` in general is that it
turns on USB auto-suspend, which can wreak havoc with input devices. To
prevent that, we then undo it for detected USB input devices.

This is a workaround for an upstream issue in Powertop, but it's a very
old issue and doesn't look like it's getting fixed anytime soon. See:

- https://github.com/fenrus75/powertop/issues/37
- https://github.com/fenrus75/powertop/issues/38
- https://github.com/fenrus75/powertop/issues/101
- https://github.com/fenrus75/powertop/issues/165

mkosi.extra/usr/bin/_kde-linux-disable-usb-autosuspend-for-input-devices

@@ -0,0 +1,16 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2025 Nate Graham <nate@kde.org>
+
+# Turn off USB auto-suspend for USB input devices, because
+# this can make them borderline unusable.
+
+set -e
+
+INPUT_DEVICE_IDS=$(find /sys/bus/usb/drivers/usbhid -regex '.*\/[0-9:.-]+' -printf '%f\n' | cut --delimiter ":" --fields 1 | sort --unique)
+
+for DEVICE_ID in $INPUT_DEVICE_IDS; do
+    DEVICE_PRETTY_NAME=$(cat /sys/bus/usb/devices/$DEVICE_ID/product)
+    echo "Disabling USB auto-suspend for device \"$DEVICE_PRETTY_NAME\" (with ID $DEVICE_ID)"
+    echo "on" >| "/sys/bus/usb/devices/$DEVICE_ID/power/control";
+done

mkosi.extra/usr/lib/systemd/system-preset/50-kde-linux.preset

@@ -15,6 +15,7 @@ enable systemd-zram-setup@zram0.service
 enable systemd-oomd.service
 enable bluetooth.service
 enable cups.service
+enable kde-linux-powertop.service
 enable tuned.service
 enable tuned-ppd.service
 enable thermald.service

mkosi.extra/usr/lib/systemd/system/kde-linux-powertop.service

@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2025 Nate Graham <nate@kde.org>
+
+[Unit]
+Description=Power tunings
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/powertop --auto-tune
+ExecStartPost=/usr/bin/_kde-linux-disable-usb-autosuspend-for-input-devices
+
+[Install]
+WantedBy=multi-user.target sleep.target